-5-
Released 2018-01-26 Bulletin No. PAXLCL-E
Drawing No. LP0567
INPUT CURRENT SIGNAL
17.4 :
RANGE
10-50MA
11.8 :
3
+LOOP
4
(-)
-LOOP
RANGE
4-20MA
(-)
5
-LOOP
K2
5.5K
S6
R
S7
2R
S8
4R
S9
8R
S10
16R
12R
(+)
S5
32R
S1
S4 S3 S2
50R
16R
8R
4R
25R
OFFSET
DIRECTION
SWITCH
(+)
ADD
(-)
SUBTRACT
FINE
OFFSET
ADJUST
25K
R
+V -V
R
CURRENT
SHUNT
RESISTORS
K2
V
II
V
FINE SPAN ADJ.
5K
SPAN=150
SPAN=0
INPUT SIGNAL SCALING CIRCUIT
BUFFER
AMP
SPAN ADJUST
OFFSET ADJUST
A/D
VOLTMETER
CIRCUIT
S3 S2 S1
DIGITAL DISPLAY
DECIMAL
POINT
SWITCHES
DIGITAL VOLTMETER
O
V
1400
700
350
175
2100
1050
525
260
130
mA
S4
SWITCH
ANNUNCIATOR
BACKLIGHT
PAXCL SCHEMATIC
4.0SCALINGTHEMETER
DESCRIPTIONOFOPERATION
The PAX Lite Current Loop Meter consists of a digital volt meter
combined with an analog scaling circuit (shown above). The unit was
designed primarily for use with 4-20 mA and 10-50 mA current loop signal
circuits. However, it can also be adapted to other current ranges, such as
0-50 mA, 0-20 mA, 0-10 mA, and in a great many applications it can be
used even with 0-5 mA and 1-5 mA current loops. In addition, input
current can be reversed in polarity resulting in negative numerical
readout with a minus (-) sign displayed. Input terminals 3 and 4 are
connected in series with 10-50 mA current loops, and Terminal 3 and 5
are series connected with 4-20 mA loops. In either case, the voltage drop
generated across the shunt resistor(s) ranges from approximately 0.12 V
min. (@ 4 or 10 mA) to 0.59 V max. (@ 20 or 50 mA). The buffer amplifier
(K1) conditions and filters the input signal voltage and applies it to the
input of the scaling circuit. The procedure for scaling PAX Lite Current
Loop Meters is simplified by dividing the scaling process into two
separate components, span adjustments and offset adjustments which
are defined in the following discussion.
SPANADJUSTMENTS
Span is defined as the numerical range that the display traverses,
disregarding decimal points, when the input signal current is varied from
minimum (4 or 10 mA) to maximum (20 or 50 mA). For example, if a unit
is to display 25.0 @ 4 mA and 100.0 @ 20 mA, the span is 750 (the
difference between 250 and 1000). Had the minimum display been -25.0
@ 4 mA and +100.0 @ 20 mA, the span would be 1250 (1000 - (-250) =
1250). (Note: the terms “GAIN”, “SCALE”, and “SENSITIVITY” are also
frequently used interchangeably with the term “SPAN.”) The PAX Lite
Current Loop Meter can be set up over
a very wide span range by means of the
coarse DIP switches S6-S10, and the
fine screwdriver adjustment pot, located
at the back cover. The coarse span
switches add parallel input resistors to
the summing amplifier (K2), thereby
increasing its gain, or sensitivity, as
more summing resistors are added.
Effectively, adding more parallel input
resistors, increases the slope of the
transfer curve (at right) and increases
the numerical readout for a given input signal current change. The input
summing resistor values are weighted in a binary progression, so they
can be switched in combinations to give 32 discrete steps of span. The
fine adjust control brackets these coarse steps and can be adjusted to
the exact span needed.
The approximate span contributed by each switch is shown on the rear
label. These values are based on the standard current-loop spans of 4 to
20 mA (16 mA current variation) and 10-50 mA (40 mA current variation).
In other words, if S7 only is turned “ON”, the numerical readout will
display a change approximately 1050 for a current swing of 16 mA (4-20
mA input) or 40 mA (10-50 mA input). If S8 were also turned “ON”, the
numerical readout would swing approximately 1575 (1050 for S7 + 525
for S8) for the same signal current variation. The fine control has a
continuous span range of approximately 0-150.
OFFSETADJUSTMENTS
In the foregoing discussion of span, the transfer curves were shown as
“ZERO-BASED”, i.e., the numerical readout displays “0” when the
signal current goes to zero. With current loop ranges such as 0-5 or 0-10,
or 0-20 mA, and with Bi-Polar (+/-) signals, this is often the desired
condition. However, with 4-20 and 10-50 mA current loops, the minimum
current level of 4 or 10 mA usually
represents the zero level of the
parameter being displayed. There
are also many applications where
the minimum (or zero level)
represents some value that does
not fall on a zero based transfer
curve. To accommodate non-zero
based applications, the PAX Lite
Current Loop Meter has provisions
for offsetting the transfer curve
over a wide range. Essentially,
offset moves the transfer curve up
or down to change its intercept with the numerical readout axis, but it
does not change the slope (SPAN) of the transfer curve. In the PAX Lite
Current Loop Meter, offset is accomplished by adding (or subtracting) a
constant at the input of the summing amplifier (K2). This offset constant is
summed in with a switched binary resistor network and a fine adjust
offset control in a similar manner to that used for span adjustment.
Switches S2-S5 can be turned on in combinations to give 16 different
coarse offset levels. Each switch is labeled to show the approximate
amount of offset contributed when it is turned “ON”. Switch 1 selects the
polarity of the switched-in offset value and allows offsetting the transfer
curve “UP” (adding the offset constant) or “DOWN” (subtracting). The fine
offset control has a numerical readout range of ±100 and brackets all the
coarse switched ranges.turned on in combinations to give 16 different
coarse offset levels. Each switch is labeled to show the approximate
amount of offset contributed when it is turned “ON.” Switch 1 selects the
polarity of the switched-in offset value and allows offsetting the transfer
curve “UP” (adding the offset constant) or “DOWN” (subtracting). The
offset adjust pot has a numerical readout range of +/-100 and brackets all
the coarse switched ranges.
-500
NUMERICAL READOUT
0
4 8 12 16 20
+500
+1000
+1500
+1999
INPUT SIGNAL CURRENT
Span Adjustment
SPAN 525 (S8 ON)
SPAN 2100 (S6 ON)
SPAN 1050 (S7 ON)
-500
+500
0
+1000
+1500
+1999
INPUT SIGNAL CURRENT
+500 OFFSET
"ZERO BASE" NO OFFSET
-300 OFFSET
-300
OFFSET
+500
OFFSET